Brainbox outspoken in row (5)
I believe the answer is:
scull
'row' is the definition.
(sculling is a kind of rowing)
'brainbox outspoken' is the wordplay.
'brainbox' becomes 'skull' (the brain is contained by the skull).
'outspoken' shows a homophone (sound like).
'skull' is a homophone of 'SCULL'.
'in' is the link.
(Other definitions for scull that I've seen before include "Light rowing boat" , "A single oar" , "Small oar" , "Blade" , "Row with two oars" .)